The use of optical devices for the protection against counterfeiting, illegal tampering and product protection in general is now a well established art.
Due to increased fraud and counterfeit, novel anti-counterfeit measures are constantly required. For many years holograms have been the preferred security technology. Meanwhile, this technology is more than 30 years old and therefore well known and widespread. Holographic foils may even be found in every gift shop today. This situation represents a security risk since many people have access to the hologram technology.
It is thus most desirable to extend the palette of security devices by novel security features, which are clearly distinguishable from holographic devices. Examples of such new devices are alternative optically variable devices (OVD). OVDs are devices that change their appearance, such as brightness, contrast or colour, as the viewing angle or illumination angle is changed. Prominent representative colorshift OVDs are cholesteric or interference films, including optical devices based on flakes of such films. Both exhibit a pronounced colorshift as the device is tilted away from a perpendicular angle of view.
Colorshift effects due to the interference of light at thin optical films have a long tradition in the history of modern thin film components (e.g. J. A. Dobrowolski, “Optical thin-film security devices”, in “Optical Document Security” ed. by R. L. van Renesse, Artechouse Boston 1998). Many different compositions of layered thin-film systems are possible. The reflection or transmission spectra are shifted towards the short wavelength side as the incidence angle increases. Multi-layer thin-film systems, often combinations of dielectric and metallic layers, are also possible with dielectric materials only. In this case, thin-films of different index of refraction are required.
Security devices based on either thin interference films or on flakes of such films are commercially available today. Examples can for instance be found in U.S. Pat. No. 5,084,351.
Other approaches are scattering devices. The use of isotropic and even more anisotropic scattering effects in OVDs can enhance the optical attractiveness significantly. Especially anisotropic light scattering is a helpful means to generate viewing angle sensitive devices. FIGS. 1.1 and 1.2 illustrate isotropic and anisotropic light scattering, respectively.
The reflection at an isotropically structured surface, such as a newsprint or most surfaces encountered in household articles, is such that no azimuthal direction is preferred. As indicated in FIG. 1.1, collimated incoming light 1 is redirected at the scattering surface 2 into new outgoing directions 3 with a characteristic axial-symmetric output light distribution and a characteristic divergence angle 4.
An anisotropically structured surface however reflects light in a pronounced way into certain directions and suppresses light in other directions. In FIG. 1.2, collimated incoming light 1 impinges on an anisotropically scattering surface 5 and is redirected into new outgoing directions 6 with a characteristic output light distribution 7, which depends on the corresponding azimuthal angle 8, 8′.
In the context of the present invention, the term anisotropy direction shall mean a local symmetry axis within the plane of a layer, for example the direction along grooves or valleys of a microstructure.
If a surface comprises a pattern of anisotropic structures with locally differing anisotropy direction, like the directions 10, 11 in FIG. 2, then the individual areas of the pattern reflect incoming light into different directions. The pattern may then be recognized by oblique observation or by using obliquely incident light.
A known method of manufacturing anisotropic scattering films with patterned anisotropy is described in the international patent application WO-01/29148, the content of which is incorporated hereby by reference. The method makes use of the so called monomer corrugation (MC) technology. It relies on the fact that phase separation of special mixtures or blends applied to a substrate is induced by crosslinking, for instance by exposure to ultraviolet radiation. The subsequent removal of non-crosslinked components leaves a structure with a specific surface topology. The term MC-layer is used for layers prepared according to this technology. The topology may be made anisotropic by the alignment of an underlying alignment layer. By using a patterned alignment layer, it is possible to create a patterned anisotropically scattering surface topology.
WO-2006/007742 discloses methods to produce modified MC-layers and layer structures, which generate pastel-colored appearance under certain observation angles.
WO07/131,375 discloses methods to generate optically effective surface relief microstructures by first manufacturing a mask comprising an image of the microstructure and then copying the image in a second step to a resin or resist to generate the optically effective surface relief microstructure. The content of WO07/131,375 is hereby incorporated by reference. A drawback of the methods disclosed in WO07/131,375 is that the number of process steps is high, which does not only increase production time but also decreases the manufacturing yield.